1. Field of the Invention
The invention relates to an optical record carrier which having an information structure arranged along tracks, which tracks are situated substantially concentrically around a centre of rotation, the the tracks having a successin of first and second annular zones which can be distinguished from each other and which are situated concentrically around the center of rotation.
The invention also relates to an apparatus for reading such a record carrier, which apparatus comprises drive means for rotating the information carrier about the centre of rotation, a radiation source producing a read beam, an objective system for focussing the read beam to form a scanning spot on the record carrier and subsequently directing the read beam to a radiation-sensitive detection system, the scanning spot being radially movable in the plane of rotation of the record carrier under control of a radial actuating device, and the detection system for reading the information structure converting the modulation of the read beam produced by the structure on the record carrier into an electric signal.
2. Description of the Related Art
Such a record carrier and apparatus are known from published Netherlands Patent Application No. 7510035 (PHN 8118), which correspond to pending U.S. application Ser. No. 128,417, filed Mar. 10, 1980. In the known record carrier the first zones differe from the second zones in that the track spacing in the first zones differs from that in the second zones. The known apparatus comprises an auxiliary radiation source for producing an additional address beam which projects an address spot on the record carrier, which spot covers a plurality of adjacent tracks. When the address spot and a scanning spot which is which is locked to the address beam move in a radial direction in such a way that they pass a number of annular zones, the different track spacings cause a reflected first-order diffraction sub-beam of the additional address beam to move to and fro over associated detectors. This to-and-fro movement is detected to provide a counting signal which indicates how many annular zones have been traversed by the address spot. This enables a specific area to be addressed rapidly and correctly, resulting in very fast access time to locate an arbitrary track.
However, the known record carrier has the disadvantage that an additional auxiliary radiation source is necessary for addressing the areas. Moreover, the varying track spacing complicates the manufacture of the record carriers. In addition, in the case of a blank still uninscribed record carrier without performed grooves addressing is impossible owing to the absence of tracks.